Quantum Sensing of Copper-Phthalocyanine Electron Spins via NV Relaxometry

TL;DR

This paper demonstrates how shallow NV centers in diamond can be used to probe and characterize the electron spin ensemble of copper phthalocyanine molecules at room temperature, revealing detailed spin dynamics and interactions.

Contribution

It introduces a novel NV relaxometry method to analyze molecular spin systems, providing new insights into their decoherence mechanisms and parameters at room temperature.

Findings

Electron-electron interactions dominate CuPc decoherence.

NV relaxometry can estimate NV depth with ~1 nm accuracy.

Hyperfine spectrum confirms NV-CuPc interaction.

Abstract

Molecular spin systems are promising candidates for quantum information processing and nanoscale sensing, yet their characterization at room temperature remains challenging due to fast spin decoherence. In this work, we use $T_1$ relaxometry of shallow nitrogen-vacancy (NV) centers in diamond to probe the electron spin ensemble of a polycrystalline copper phthalocyanine (CuPc) thin film. In addition to unequivocally identifying the NV-CuPc interaction thanks to its hyperfine spectrum, we further extract key parameters of the CuPc spin ensemble, including its correlation time and local lattice orientation, that cannot be measured in bulk electron resonance experiments. The analysis of our experimental results confirms that electron-electron interactions dominate the decoherence dynamics of CuPc at room temperature. Additionally, we demonstrate that the CuPc-enhanced NV relaxometry can serve as a robust method to estimate the NV depth with $\sim1$~nm precision. Our results establish NV centers as powerful probes for molecular spin systems, providing insights into molecular qubits, spin bath engineering, and hybrid quantum materials, and offering a potential pathway toward their applications such as molecular-scale quantum processors and spin-based quantum networks.